27 feb 2013 lab no 3 hardness

Lab # 3Determine the

Hardness of Water

Lokhaze AliVisiting Faculty

NICE, SCEE, NUST

Lab # 3 Water Hardness 2

Water Hardness

Water's hardness is determined by the concentration of multivalent cations in water. There are many different divalent salts, however, calcium and magnesium are the most common sources of water hardness.


Water Hardness

• Water is said to be “hard” when it contains Ca+2 and Mg+2 ions. These ions react with soap to form an insoluble substance called “soap scum”

2C17H35COONa + Ca+2 (C17H35COO)2Ca + 2Na+

Sodium Stearate Calcium Stearate

• Hardness of water is caused by the presence of multivalent cations such as Ca+2, Mg+2, Fe+2, Al+3, Mn+2, Ba+2, Sr+2 in water.


Total Hardness

Calcium and Magnesium Hardness

Total Hardness = Ca+2 Hardness + Mg+2 Hardness


Temporary/Permanent Hardness

a) Carbonate Hardness/Temporary HardnessThe part of total hardness that is caused by Calcium bicarbonate and Magnesium bicarbonateCa(HCO3)2 Ca+2 + 2HCO3

Mg(HCO3)2 Mg+2 + 2HCO3

b) Non-Carbonate Hardness/Permanent HardnessDue to the presence of chlorides or sulphates of calcium and magnesium such as MgCl2, MgSO4 and CaCl22CaCl2 2Ca+2 + 2Cl-

MgSO4 Mg+2 + SO4-2


Temporary Hardness

Temporary hardness can be removed by boiling

Ca(HCO3) aq + Heat CO2 + H2O + CaCO3

Mg(HCO3) aq + Heat CO2 + H2O + MgCO3

No health-based guideline value is proposed for hardness. However, the degree of hardness in water may affect its acceptability to the consumer in terms of taste and scale deposition


Water Hardness

mg/L as CaCO3 Degree of Hardness

0 - 50 Soft

50 - 100 Moderately Soft

100 - 150 Slightly Hard

150 - 200 Moderately Hard

> 200 Hard

>300 Very Hard


Concerns

• Scaling• Cleaning• Public Health


Methods of Analysis

Titrimetric MethodBurette

• A piece of glassware designed to deliver known amounts of liquid into another container


EDTA

• Ethylenediaminetetracetic acid• Chelating agent

Substance whose molecules can form several bonds to a single metal ionMulti-dentate ligands

• Many clawed; holding onto the metal ion to form a very stable complex

• 4-6 bonds


Water HardnessDetermination with EDTA

• H2EDTA2- binds to a Ca2+ ion by forming four special covalent bonds called coordinate covalent bonds.

H2EDTA2-

Ca2++ + 2 H+

Ca2+ ion chelated by

EDTA


EBT

Eriochrome BlackIndicator


Procedure (Titration of Sample)

• Use a volumetric pipet to dispense 50.00 mL of hard water sample into a 250 mL flask.

• Add 5 mL of pH 10 buffer, 2 drops of Eriochrome Black T indicator, and 15 drops of 0.03 M MgCl2.

• Titrate the solution with EDTA from your burette to the blue endpoint you observed when titrating the blank.

Thank You

Technology

27 feb 2013 lab no 3 hardness